1. Field of the Invention
This invention relates to load switching circuits and particularly to an electronic simulation of a latching relay.
2. Description of the Prior Art
Many different prior art apparatuses or systems have been proposed for automatically switching a load to a standby power source upon the occurrence of some operating condition. Generally, each of such load-switching apparatuses automatically switches a load from a primary power supply to a standby or emergency power supply when the primary power supply has totally or partially failed and automatically switches the load back to the primary power supply when the primary power supply has been fully restored.
In a first type of load-switching apparatus the standby power supply is a battery which is not utilized to supply current to the load until there is a primary power failure. Thus, there are two separate power supplies, electrically-isolated from each other, which selectively supply energy to the load at different times, but not both at the same time. Examples of this first type of apparatus are described in U.S. Pat. Nos.: 3,308,306; 3,356,857; 3,390,307; 3,596,106; 4,044,268; and 4,209,710.
In a second type of load-switching apparatus a standby battery is connected across the load at all times to cooperate with a primary DC energy source in meeting the current demands of the load. During normal operation the primary DC energy source supplies current to the load, as well as a charging current to the battery when the battery charge level is low. Upon failure of the DC energy source, the battery alone supplies current to the load. Upon restoration of the DC energy source, normal operation is resumed. Examples of this second type of apparatus are described in U.S. Pat. Nos. 1,393,940 and 3,599,073.
In all of the above-described apparatuses, the switching of the load from a primary power supply to a standby power supply during a primary power loss and back to the primary power supply after the primary power is restored can be repeated each time that the primary power is lost and then restored. None of the above-described apparatuses teaches or suggests an electronic latching circuit which electrically isolates the battery from the load in an electronic equipment during the time that the equipment is being shipped or stored and automatically permanently switches or connects the battery to the load the first time that the equipment is put into operation and turned on after its shipment or storage.